Exhaust gas purification system and exhaust gas purification method

ABSTRACT

An exhaust gas purification system and an exhaust gas purification method are disclosed. The exhaust gas purification system is provided with: an injector for injecting urea water, said injector being installed between a combustion device and a selective catalytic reduction (SCR) catalyst; a first gas sensor which is installed downstream of the SCR catalyst, and which detects the NO concentration and the NH 3  concentration in exhaust gas outputted from the SCR catalyst; and an opening amount control means for controlling the opening amount of the injector for injecting urea water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Application No.PCT/JP2017/022947 filed on Jun. 22, 2017, which is based upon and claimsthe benefit of priority from Japanese Patent Application No. 2016-124422filed on Jun. 23, 2016, the contents all of which are incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification system andan exhaust gas purification method using a gas sensor, which is capableof measuring respective concentrations of a plurality of targetcomponents in a gas to be measured.

BACKGROUND ART

Conventionally, an NOx sensor (a serially arranged two-chamber type NOxsensor) having a serially arranged two-chamber structure, and a NOxmeasurement method using the same (for example, refer to JapaneseLaid-Open Patent Publication No. 2015-200643), and a mixed potentialtype, or a variable resistance type NO₂ sensor in which an oxidesemiconductor electrode is used, or an NH₃ sensor are known (forexample, refer to Japanese Laid-Open Patent Publication No. 2013-068632and Japanese Laid-Open Patent Publication No. 2009-243942).

Further, a method of measuring an NH₃ concentration using a mixedpotential of an oxide semiconductor electrode is known. This method is amethod in which the NOx concentration is measured by another sensor, andin the case that NO and NO₂ are not present, the mixed potential of theoxide semiconductor electrode is used as is, whereas, in the case thatNO and NO₂ are present, a correction is added to the mixed potential ofthe oxide semiconductor electrode (see, for example, Japanese Laid-OpenPatent Publication No. 2009-511859 (PCT)).

SUMMARY OF INVENTION

In recent years, there is a tendency for regulations in regard to CO₂emission levels to be strengthened, and the adoption rate of dieselvehicles is increasing in respective countries. Diesel engines usinglean combustion possess a disadvantage in that it is difficult to purifyNOx in exhaust gas that contains an excessive amount of oxygen insteadof a small amount of CO₂ emissions. For this reason, similar tostrengthening regulations concerning CO₂ emissions, regulationsconcerning NOx emissions are also being strengthened. Currently, aselective reduction type catalyst system (hereinafter referred to as anSCR system) which can perform NOx purification without impairing CO₂emission, that is, without a loss in fuel consumption, occupies themainstream of NOx purification. In such an SCR system, injected urea isreacted with exhaust gas to produce ammonia, and the ammonia and NOx arereacted and are thereby decomposed into N₂ and O₂. In the SCR system, inorder that the NOx purification efficiency is made close to 100%, it isnecessary to increase the injected amount of urea. However, if theinjected amount of urea is increased, unreacted ammonia may bedischarged into the atmosphere. Therefore, a sensor capable ofdistinguishing between NOx and ammonia is required.

Furthermore, in the United States, preparations are being advanced withrespect to obligations for individual failure diagnosis of oxidationcatalysts (hereinafter referred to as DOC catalysts), diesel particulatefilters (hereinafter referred to as DPF), and selective reduction typecatalysts (hereinafter referred to as SCR catalysts). Although failurediagnosis of DPF and SCR catalysts are possible with existing PM sensorsand NOx sensors, an effective failure diagnosis technique has not beendiscovered with respect to DOC catalysts. Currently, a method ofmeasuring an amount of hydrocarbons (hereinafter referred to as HC)leaking downstream from a DOC catalyst at a low temperature of 200° C.or less, and a method of judging a failure from a ratio of NO and NO₂that are discharged downstream from a DOC catalyst are recommended. Inparticular, in the ratio of NO and NO₂, since a reduction in NO₂ occursearlier than an increase in the HC outflow amount, such a method isexpected to be a safer method of fault diagnosis. For this purpose, asensor that is capable of distinguishing between NO and NO₂ is demanded.

In the NOx sensor and the NOx measurement method described in theaforementioned Japanese Laid-Open Patent Publication No. 2015-200643,NO, NO₂, and NH₃ are converted into NO, and after conversion thereof,the NO is decomposed, and a generated amount or a concentration of 02 ismeasured. Therefore, although the total amount of NO, NO₂, and NH₃ canbe measured, it is not possible to distinguish between these respectivecomponents.

Although the oxide semiconductor electrodes described in JapaneseLaid-Open Patent Publication No. 2013-068632 and Japanese Laid-OpenPatent Publication 2009-243942 are excellent in terms of the selectivityof NO and NO₂, on the other hand, since the sensitivity outputcharacteristics with respect to NO and NO₂ are opposite in polarity,under an atmosphere in which both NO and NO₂ coexist, it has not beenpossible to correctly measure the concentration of NO or NO₂.

In the sensor described in Japanese Laid-Open Patent Publication No.2009-511859 (PCT), it is difficult to accurately measure an NH₃concentration over a prolonged time period, due to the instability ofthe oxide semiconductor electrode within the exhaust gas, and the weakadhesion strength with the substrate.

The present invention has been devised taking into consideration theaforementioned problems, and has the object of providing an exhaust gaspurification system and an exhaust gas purification method capable ofpurifying NOx and reducing an exhaust amount of NH₃ accurately over aprolonged time period by using a gas sensor which is possible toaccurately measure over a prolonged time period the concentration of anon-combusted component such as exhaust gas, and a plurality ofcomponents (for example, NO, NO₂, and NH₃) that coexist in the presenceof oxygen.

[1] An exhaust gas purification system according to a first aspect ofthe present invention includes a urea water injector disposed between acombustion device and a selective reduction type catalyst (hereinafterreferred to as SCR catalyst), a first gas sensor disposed downstream ofthe SCR catalyst and adapted to detect an NO concentration and an NH₃concentration in an exhaust gas downstream of the SCR catalyst, and adegree of opening control unit adapted to control a degree of opening ofthe urea water injector, the degree of opening control unit is adaptedto control the degree of opening of the urea injector in a direction toopen as time passes on a condition that the NO concentration has reacheda predetermined first threshold value and control the degree of openingof the urea injector in a direction to close as time passes on acondition that the NH₃ concentration has reached a predetermined secondthreshold value.

[2] In the first aspect of the present invention, the first thresholdvalue is set to a value that is higher than the NO concentration atrespective equivalent points of the NO concentration and the NH₃concentration, and the second threshold value is set to a value that ishigher than the NH₃ concentration at the respective equivalent points.

[3] In the first aspect of the present invention, the exhaust gaspurification system further includes a deterioration detection unit fora selective reduction type catalyst (hereinafter referred to as SCRdeterioration detection unit), the SCR deterioration detection unitbeing adapted to compare a ratio of the NO concentration to the NH₃concentration (NO/NH₃) at a predetermined degree of opening with aprescribed value, and judge deterioration of the SCR catalyst on acondition that the ratio exceeds the prescribed value.

[4] In the first aspect of the present invention, a first gas sensor mayinclude a sensor element having a structural body made up from a solidelectrolyte that exhibits at least oxygen ion conductivity, a gasintroduction port formed in the structural body and into which a gas tobe measured is introduced, an oxygen concentration adjustment chamberformed in the structural body and communicating with the gasintroduction port, and a measurement chamber formed in the structuralbody and communicating with the oxygen concentration adjustment chamber,an oxygen concentration control unit adapted to control the oxygenconcentration in the oxygen concentration adjustment chamber, atemperature control unit adapted to control a temperature of the sensorelement, and a specified component measurement unit adapted to measure aconcentration of a specified component in the measurement chamber, thefirst gas sensor further comprising a preliminary adjustment chamberprovided within the structural body between the gas introduction portand the oxygen concentration adjustment chamber, and communicating withthe gas introduction port, a preliminary oxygen concentration controlunit adapted to control the oxygen concentration inside the preliminaryadjustment chamber, a drive control unit adapted to control driving andstopping of the preliminary oxygen concentration control unit, and atarget component acquisition unit adapted to acquire the NOconcentration and the NH₃ concentration, on the basis of a differencebetween a sensor output from the specified component measurement unit ata time that the preliminary oxygen concentration control unit is driven,and a sensor output from the specified component measurement unit at atime that the preliminary oxygen concentration control unit is stopped,and one of the respective sensor outputs.

[5] In this case, the target component acquisition unit may utilize afirst map. In the first map, there is recorded a relationship, which ismeasured experimentally in advance, between the NO concentration and theNH₃ concentration respectively for each of points specified by thesensor output from the specified component measurement unit at a time ofstopping the preliminary oxygen concentration control unit, and adifference in the sensor outputs from the specified componentmeasurement unit at times of driving and stopping the preliminary oxygenconcentration control unit. The target component acquisition unitobtains the respective concentrations of NO and NH₃ by comparing withthe first map the sensor output from the specified component measurementunit at the time of stopping the preliminary oxygen concentrationcontrol unit during actual use, and the difference in the sensor outputsfrom the specified component measurement unit at the times of drivingand stopping the preliminary oxygen concentration control unit.

[6] Alternatively, the target component acquisition unit may obtain theNO concentration in the following manner. More specifically, the targetcomponent acquisition unit obtains the NH₃ concentration correspondingto a difference in the sensor outputs from the specified componentmeasurement unit at times of driving and stopping the preliminary oxygenconcentration control unit during actual use, on the basis of arelationship, which is measured experimentally in advance, between theNH₃ concentration and the difference in the sensor outputs from thespecified component measurement unit at the times of driving andstopping the preliminary oxygen concentration control unit. In addition,the target component acquisition unit may obtain the NO concentration byan operation of subtracting the NH₃ concentration, which was obtainedbeforehand from the difference in the sensor outputs, from a total NOconcentration in which all of the concentrations of NO and NH₃ obtainedfrom the sensor output at a time of stopping the preliminary oxygenconcentration control unit are converted into NO.

[7] In the first aspect of the present invention, the exhaust gaspurification system may further include an oxidation catalyst(hereinafter referred to as DOC catalyst) disposed between thecombustion device and the urea water injector, a second gas sensordisposed between the oxidation catalyst and the urea water injector andadapted to detect an NO concentration and an NO₂ concentration in anexhaust gas downstream of the DOC catalyst, and a deteriorationdetection unit for the oxidation catalyst, the DOC deteriorationdetection unit being adapted to compare a ratio of the NO concentrationto the NO₂ concentration (NO/NO₂) with a second prescribed value, andjudge deterioration of the DOC catalyst on a condition that the ratioexceeds the second prescribed value.

[8] In this case, a second gas sensor may include a sensor elementhaving a structural body made up from a solid electrolyte that exhibitsat least oxygen ion conductivity, a gas introduction port formed in thestructural body and into which a gas to be measured is introduced, anoxygen concentration adjustment chamber formed in the structural bodyand communicating with the gas introduction port, and a measurementchamber formed in the structural body and communicating with the oxygenconcentration adjustment chamber, an oxygen concentration control unitadapted to control the oxygen concentration in the oxygen concentrationadjustment chamber, a temperature control unit adapted to control atemperature of the sensor element, and a specified component measurementunit adapted to measure a concentration of a specified component in themeasurement chamber, the second gas sensor further comprising apreliminary adjustment chamber provided within the structural bodybetween the gas introduction port and the oxygen concentrationadjustment chamber, and communicating with the gas introduction port, apreliminary oxygen concentration control unit adapted to control theoxygen concentration inside the preliminary adjustment chamber, a drivecontrol unit adapted to control driving and stopping of the preliminaryoxygen concentration control unit, and a target component acquisitionunit adapted to acquire the NO concentration and the NH₃ concentration,on the basis of a difference between a sensor output from the specifiedcomponent measurement unit at a time that the preliminary oxygenconcentration control unit is driven, and a sensor output from thespecified component measurement unit at a time that the preliminaryoxygen concentration control unit is stopped, and one of the respectivesensor outputs.

[9] Further, the target component acquisition unit may utilize a secondmap. In the second map, there is recorded a relationship, which ismeasured experimentally in advance, between the NO concentration and theNO₂ concentration respectively for each of points specified by thesensor output from the specified component measurement unit at a time ofstopping the preliminary oxygen concentration control unit, and adifference in the sensor outputs from the specified componentmeasurement unit at times of driving and stopping the preliminary oxygenconcentration control unit. The target component acquisition unitobtains the respective concentrations of NO and NO₂ by comparing withthe second map the sensor output from the specified componentmeasurement unit at the time of stopping the preliminary oxygenconcentration control unit during actual use, and the difference in thesensor outputs from the specified component measurement unit at thetimes of driving and stopping the preliminary oxygen concentrationcontrol unit.

[10] Alternatively, the target component acquisition unit may obtain theNO concentration in the following manner. More specifically, the targetcomponent acquisition unit obtains the NO₂ concentration correspondingto a difference in the sensor outputs from the specified componentmeasurement unit at times of driving and stopping the preliminary oxygenconcentration control unit during actual use, on the basis of arelationship, which is measured experimentally in advance, between theNO₂ concentration and the difference in the sensor outputs from thespecified component measurement unit at the times of driving andstopping the preliminary oxygen concentration control unit. In addition,the target component acquisition unit may obtain the NO concentration byan operation of subtracting the NO₂ concentration, which was obtainedbeforehand from the difference in the sensor outputs, from a total NOconcentration in which all of the concentrations of NO and NO₂ obtainedfrom the sensor output at a time of stopping the preliminary oxygenconcentration control unit are converted into NO.

[11] An exhaust gas purification method according to a second aspect ofthe present invention purifies an exhaust gas using an exhaust gaspurification system including a urea water injector disposed between acombustion device and a SCR catalyst, a first gas sensor disposeddownstream of the SCR catalyst and adapted to detect an NO concentrationand an NH₃ concentration in an exhaust gas downstream of the SCRcatalyst, and a degree of opening control unit adapted to control adegree of opening of the urea water injector, including the steps of:

controlling the degree of opening of the urea injector in a direction toopen as time passes on a condition that the NO concentration has reacheda predetermined first threshold value; and

controlling the degree of opening of the urea injector in a direction toclose as time passes on a condition that the NH₃ concentration hasreached a predetermined second threshold value.

[12] In the second aspect of the present invention, the method furtherincludes the step of comparing a ratio of the NO concentration to theNH₃ concentration (NO/NH₃) at a predetermined degree of opening with aprescribed value, and judging deterioration of the SCR catalyst on acondition that the ratio exceeds the prescribed value.

[13] In the second aspect of the present invention, the exhaust gaspurification system may include an oxidation catalyst disposed betweenthe combustion device and the urea water injector, and a second gassensor adapted to detect an NO concentration and an NO₂ concentration inan exhaust gas downstream of the DOC catalyst, and the method mayinclude the step of comparing a ratio of the NO concentration to the NO₂concentration (NO/NO₂) with a second prescribed value, and judgingdeterioration of the oxidation catalyst on a condition that the ratioexceeds the second prescribed value.

In accordance with the exhaust gas purification system and the exhaustgas purification method according to the present invention, since a gassensor capable of accurately measuring over a prolonged time period theconcentration of a non-combusted component such as exhaust gas, and aplurality of components (for example, NO, NO₂, and NH₃) that coexist inthe presence of oxygen, it is possible to purify NOx and reduce anexhaust amount of NH₃ accurately over a prolonged time period.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view in which there is shown one structuralexample of a first gas sensor and a second gas sensor used in theexhaust gas purification system according to the embodiment;

FIG. 2 is a configuration diagram schematically showing a first gassensor;

FIG. 3 is an explanatory diagram schematically showing reactions in apreliminary adjustment chamber, an oxygen concentration adjustmentchamber, and a measurement chamber, for a case in which a preliminarypump cell is turned off in the first gas sensor;

FIG. 4 is an explanatory diagram schematically showing reactions insidethe preliminary adjustment chamber, the oxygen concentration adjustmentchamber, and the measurement chamber, for a case in which thepreliminary pump cell is turned on in the first gas sensor;

FIG. 5 is a view graphically showing a first map utilized by the firstgas sensor;

FIG. 6 is an explanatory diagram showing the first map utilized by thefirst gas sensor in the form of a table;

FIG. 7 is a flowchart showing an example of a process of measuring NOand NH₃ by the first gas sensor;

FIG. 8 is a configuration diagram schematically showing a second gassensor;

FIG. 9 is an explanatory diagram schematically showing reactions in apreliminary adjustment chamber, an oxygen concentration adjustmentchamber, and a measurement chamber, for a case in which a preliminarypump cell is turned off in the second gas sensor;

FIG. 10 is an explanatory diagram schematically showing reactions insidethe preliminary adjustment chamber, the oxygen concentration adjustmentchamber, and the measurement chamber, for a case in which thepreliminary pump cell is turned on in the second gas sensor;

FIG. 11 is a view graphically showing a second map utilized by thesecond gas sensor;

FIG. 12 is an explanatory diagram showing the second map utilized by thesecond gas sensor in the form of a table;

FIG. 13 is a flowchart showing an example of a process of measuring NOand NO₂ by the second gas sensor;

FIG. 14 is a configuration diagram showing an exhaust gas purificationsystem according to the present embodiment;

FIG. 15 is a graph showing an SCR efficiency (%), an emission level ofNH₃ (ppm), and an NO concentration (ppm) measured by a conventional gassensor with respect to changes in an injected amount of urea water;

FIG. 16 is a graph showing an SCR efficiency (%), an emission level ofNH₃ (ppm), and a sensor output (NO output (μA)) of the first gas sensorwith respect to changes in an injected amount of urea water;

FIG. 17A is a graph showing a relationship between an injected amount ofurea water and the sensor output of the first gas sensor;

FIG. 17B is a graph separated into a sensor output (NO output) inregards to NO, and a sensor output (NH₃ output) in regards to NH₃;

FIG. 18A is a graph showing changes in NO output accompanying an elapseof time;

FIG. 18B is a graph showing changes in NH₃ output accompanying an elapseof time;

FIG. 18C is a graph showing changes in a degree of opening of a ureainjector accompanying an elapse of time;

FIG. 19 is a flowchart showing an example of a processing operation ofthe exhaust gas purification system according to the present embodiment;and

FIG. 20 is a cross-sectional view showing a structural example of amodified example of the first gas sensor and the second gas sensor.

DESCRIPTION OF EMBODIMENTS

Embodiments of an exhaust gas purification system and an exhaust gaspurification method will be presented and described below with referenceto FIGS. 1 to 20. In the present specification, the term “to” when usedto indicate a numerical range is used with the implication of includingthe numerical values written before and after the term as a lower limitvalue and an upper limit value of the numerical range.

First, a first gas sensor 10A and a second gas sensor 10B used in anexhaust gas purification system 200 (see FIG. 14) according to thepresent embodiment will be explained with reference to FIGS. 1 to 13.

As shown in FIGS. 1 and 2, the first gas sensor 10A includes a firstsensor element 12A. The first sensor element 12A includes a structuralbody 14 made up from a solid electrolyte that exhibits at least oxygenion conductivity, a gas introduction port 16 formed in the structuralbody 14 and into which a gas to be measured is introduced, an oxygenconcentration adjustment chamber 18 formed in the structural body 14 andcommunicating with the gas introduction port 16, and a measurementchamber 20 formed in the structural body 14 and communicating with theoxygen concentration adjustment chamber 18.

The oxygen concentration adjustment chamber 18 includes a mainadjustment chamber 18 a communicating with the gas introduction port 16,and an auxiliary adjustment chamber 18 b communicating with the mainadjustment chamber 18 a. The measurement chamber 20 communicates withthe auxiliary adjustment chamber 18 b.

Furthermore, the first gas sensor 10A includes a preliminary adjustmentchamber 21 provided between the gas introduction port 16 and the mainadjustment chamber 18 a within the structural body 14, and whichcommunicates with the gas introduction port 16.

More specifically, the structural body 14 of the first sensor element12A is constituted by six layers including a first substrate layer 22 a,a second substrate layer 22 b, a third substrate layer 22 c, a firstsolid electrolyte layer 24, a spacer layer 26, and a second solidelectrolyte layer 28, which are stacked in this order from a lower sideas viewed in the drawing. The respective layers are composedrespectively of an oxygen ion conductive solid electrolyte layer such aszirconia (ZrO₂) or the like.

Between a lower surface of the second solid electrolyte layer 28 and anupper surface of the first solid electrolyte layer 24 on a distal endside of the first sensor element 12A, there are provided the gasintroduction port 16, a first diffusion rate control portion 30, thepreliminary adjustment chamber 21, a second diffusion rate controlportion 32, the oxygen concentration adjustment chamber 18, a thirddiffusion rate control portion 34, and the measurement chamber 20.Further, a fourth diffusion rate control portion 36 is provided betweenthe main adjustment chamber 18 a and the auxiliary adjustment chamber 18b that make up the oxygen concentration adjustment chamber 18.

The gas introduction port 16, the first diffusion rate control portion30, the preliminary adjustment chamber 21, the second diffusion ratecontrol portion 32, the main adjustment chamber 18 a, the fourthdiffusion rate control portion 36, the auxiliary adjustment chamber 18b, the third diffusion rate control portion 34, and the measurementchamber 20 are formed adjacent to each other in a manner communicatingin this order. The portion from the gas introduction port 16 leading tothe measurement chamber 20 is also referred to as a gas flow section.

The gas introduction port 16, the preliminary adjustment chamber 21, themain adjustment chamber 18 a, the auxiliary adjustment chamber 18 b, andthe measurement chamber 20 are internal spaces provided by hollowing outthe spacer layer 26. Any of the preliminary adjustment chamber 21, themain adjustment chamber 18 a, the auxiliary adjustment chamber 18 b, andthe measurement chamber 20 is arranged in a manner so that respectiveupper parts thereof are defined by a lower surface of the second solidelectrolyte layer 28, respective lower parts thereof are defined by anupper surface of the first solid electrolyte layer 24, and respectiveside parts thereof are defined by side surfaces of the spacer layer 26.

Any of the first diffusion rate control portion 30, the third diffusionrate control portion 34, and the fourth diffusion rate control portion36 is provided as two horizontally elongated slits (in which openingsthereof have a longitudinal direction in a direction perpendicular tothe drawing). The second diffusion rate control portion 32 is providedas one horizontally elongated slit (in which an opening thereof has alongitudinal direction in a direction perpendicular to the drawing).

Further, a reference gas introduction space 38 is disposed between anupper surface of the third substrate layer 22 c and a lower surface ofthe spacer layer 26, at a position that is farther from the distal endside than the gas flow section. The reference gas introduction space 38is an internal space in which an upper part thereof is defined by alower surface of the spacer layer 26, a lower part thereof is defined byan upper surface of the third substrate layer 22 c, and a side partthereof is defined by a side surface of the first solid electrolytelayer 24. For example, oxygen or atmospheric air is introduced as areference gas into the reference gas introduction space 38.

The gas introduction port 16 is a location that opens with respect tothe external space, and the target gas to be measured is drawn into thefirst sensor element 12A from the external space through the gasintroduction port 16.

The first diffusion rate control portion 30 is a location that imparts apredetermined diffusion resistance to the gas to be measured which isintroduced from the gas introduction port 16 into the preliminaryadjustment chamber 21. Details concerning the preliminary adjustmentchamber 21 will be described later.

The second diffusion rate control portion 32 is a location that impartsa predetermined diffusion resistance to the gas to be measured which isintroduced from the preliminary adjustment chamber 21 into the mainadjustment chamber 18 a.

The main adjustment chamber 18 a is provided as a space for the purposeof adjusting an oxygen partial pressure within the gas to be measuredthat is introduced from the gas introduction port 16. The oxygen partialpressure is adjusted by operation of a main pump cell 40.

The main pump cell 40 comprises an electrochemical pump cell (mainelectrochemical pumping cell), which is constituted by a main interiorside pump electrode 42, an exterior side pump electrode 44, and anoxygen ion conductive solid electrolyte which is sandwiched between thetwo pump electrodes. The main interior side pump electrode 42 isprovided substantially over the entire surface of an upper surface ofthe first solid electrolyte layer 24, a lower surface of the secondsolid electrolyte layer 28, and side surfaces of the spacer layer 26that define the main adjustment chamber 18 a. The exterior side pumpelectrode 44 is provided in a condition of being exposed to the externalspace in a region corresponding to the main interior side pump electrode42 on the upper surface of the second solid electrolyte layer 28. Themain interior side pump electrode 42 and the exterior side pumpelectrode 44 are made of a material that weakens the reductioncapability with respect to the NOx component within the gas to bemeasured. For example, the pump electrodes are formed as porous cermetelectrodes (for example, cermet electrodes of ZrO₂ and a noble metalsuch as Pt, containing 0.1 to 30.0 wt % of Au) having rectangular shapesas viewed in plan.

The main pump cell 40 applies a first pump voltage Vp1 supplied from afirst variable power source 46 which is provided externally of the firstsensor element 12A, and by allowing a first pump current Ip1 to flowbetween the exterior side pump electrode 44 and the main interior sidepump electrode 42, it is possible to pump oxygen in the interior of themain adjustment chamber 18 a into the external space, or alternatively,to pump oxygen in the external space into the main adjustment chamber 18a.

Further, the first sensor element 12A includes a first oxygen partialpressure detecting sensor cell 50 which is an electrochemical sensorcell. The first oxygen partial pressure detecting sensor cell 50 isconstituted by the main interior side pump electrode 42, a referenceelectrode 48 sandwiched between the first solid electrolyte layer 24 andan upper surface of the third substrate layer 22 c, and an oxygen ionconductive solid electrolyte sandwiched between these electrodes. Thereference electrode 48 is an electrode having a substantiallyrectangular shape as viewed in plan, which is made from a porous cermetin the same manner as the exterior side pump electrode 44 and the like.Further, around the periphery of the reference electrode 48, a referencegas introduction layer 52 is provided, which is made from porous aluminaand is connected to the reference gas introduction space 38. Morespecifically, the reference gas in the reference gas introduction space38 is introduced to the surface of the reference electrode 48 via thereference gas introduction layer 52. The first oxygen partial pressuredetecting sensor cell 50 generates a first electromotive force V1between the main interior side pump electrode 42 and the referenceelectrode 48, which is caused by the difference in oxygen concentrationbetween the atmosphere inside the main adjustment chamber 18 a and thereference gas in the reference gas introduction space 38.

The first electromotive force V1 generated in the first oxygen partialpressure detecting sensor cell 50 changes depending on the oxygenpartial pressure of the atmosphere existing in the main adjustmentchamber 18 a. In accordance with the first electromotive force V1, thefirst sensor element 12A feedback-controls the first variable powersource 46 of the main pump cell 40. Consequently, the first pump voltageVp1, which is applied by the first variable power source 46 to the mainpump cell 40, can be controlled in accordance with the oxygen partialpressure of the atmosphere in the main adjustment chamber 18 a.

The fourth diffusion rate control portion 36 imparts a predetermineddiffusion resistance to the gas to be measured, the oxygen concentration(oxygen partial pressure) of which is controlled by operation of themain pump cell 40 in the main adjustment chamber 18 a, and is a locationthat guides the gas to be measured into the auxiliary adjustment chamber18 b.

The auxiliary adjustment chamber 18 b is provided as a space for furthercarrying out adjustment of the oxygen partial pressure by an auxiliarypump cell 54, with respect to the gas to be measured which is introducedthrough the fourth diffusion rate control portion 36, after the oxygenconcentration (oxygen partial pressure) has been adjusted beforehand inthe main adjustment chamber 18 a. In accordance with this feature, theoxygen concentration inside the auxiliary adjustment chamber 18 b can bekept constant with high accuracy, and therefore, the first gas sensor10A is made capable of measuring the NOx concentration with highaccuracy.

The auxiliary pump cell 54 is an electrochemical pump cell, and isconstituted by an auxiliary pump electrode 56, which is providedsubstantially over the entirety of the lower surface of the second solidelectrolyte layer 28 facing toward the auxiliary adjustment chamber 18b, the exterior side pump electrode 44, and the second solid electrolytelayer 28.

Moreover, in the same manner as the main interior side pump electrode42, the auxiliary pump electrode 56 is also formed using a material thatweakens the reduction capability with respect to the NOx componentwithin the gas to be measured.

The auxiliary pump cell 54, by applying a desired second voltage Vp2between the auxiliary pump electrode 56 and the exterior side pumpelectrode 44, is capable of pumping out oxygen within the atmosphereinside the auxiliary adjustment chamber 18 b into the external space, oralternatively, is capable of pumping in oxygen from the external spaceinto the auxiliary adjustment chamber 18 b.

Further, in order to control the oxygen partial pressure within theatmosphere inside the auxiliary adjustment chamber 18 b, anelectrochemical sensor cell, and more specifically, a second oxygenpartial pressure detecting sensor cell 58 for controlling the auxiliarypump, is constituted by the auxiliary pump electrode 56, the referenceelectrode 48, the second solid electrolyte layer 28, the spacer layer26, and the first solid electrolyte layer 24.

Moreover, the auxiliary pump cell 54 carries out pumping by a secondvariable power source 60, the voltage of which is controlled based on asecond electromotive force V2 detected by the second oxygen partialpressure detecting sensor cell 58. Consequently, the oxygen partialpressure within the atmosphere inside the auxiliary adjustment chamber18 b is controlled so as to become a low partial pressure that does notsubstantially influence the measurement of NOx.

Further, together therewith, a second pump current Ip2 of the auxiliarypump cell 54 is used so as to control the second electromotive force V2of the second oxygen partial pressure detecting sensor cell 58. Morespecifically, the second pump current Ip2 is input as a control signalto the second oxygen partial pressure detecting sensor cell 58, and bycontrolling the second electromotive force V2, the gradient of theoxygen partial pressure within the gas to be measured, which isintroduced through the fourth diffusion rate control portion 36 into theauxiliary adjustment chamber 18 b, is controlled so as to remainconstant at all times. When the first gas sensor 10A is used as an NOxsensor, by the actions of the main pump cell 40 and the auxiliary pumpcell 54, the oxygen concentration inside the auxiliary adjustmentchamber 18 b is maintained at a predetermined value with high accuracyfor each of the respective conditions.

The third diffusion rate control portion 34 imparts a predetermineddiffusion resistance to the gas to be measured, the oxygen concentration(oxygen partial pressure) of which is controlled by operation of theauxiliary pump cell 54 in the auxiliary adjustment chamber 18 b, and isa location that guides the gas to be measured into the measurementchamber 20.

Measurement of the NOx concentration is primarily performed byoperations of a measurement pump cell 61 provided in the measurementchamber 20. The measurement pump cell 61 is an electrochemical pump cellconstituted by a measurement electrode 62, the exterior side pumpelectrode 44, the second solid electrolyte layer 28, the spacer layer26, and the first solid electrolyte layer 24. The measurement electrode62 is provided, for example, directly on the upper surface of the firstsolid electrolyte layer 24 inside the measurement chamber 20, and is aporous cermet electrode made of a material whose reduction capabilitywith respect to the NOx component within the gas to be measured ishigher than that of the main interior side pump electrode 42. Themeasurement electrode 62 also functions as an NOx reduction catalyst forreducing NOx existing within the atmosphere above the measurementelectrode 62.

The measurement pump cell 61 is capable of pumping out oxygen that isgenerated by the decomposition of nitrogen oxide within the atmospherearound the periphery of the measurement electrode 62 (inside themeasurement chamber 20), and can detect the generated amount as ameasurement pump current Ip3, or stated otherwise, as the sensor output.

Further, in order to detect the oxygen partial pressure around theperiphery of the measurement electrode 62 (inside the measurementchamber 20), an electrochemical sensor cell, and more specifically, athird oxygen partial pressure detecting sensor cell 66 for controllingthe measurement pump, is constituted by the first solid electrolytelayer 24, the measurement electrode 62, and the reference electrode 48.A third variable power source 68 is controlled based on a thirdelectromotive force V3 detected by the third oxygen partial pressuredetecting sensor cell 66.

The gas to be measured, which is introduced into the auxiliaryadjustment chamber 18 b, reaches the measurement electrode 62 inside themeasurement chamber 20 through the third diffusion rate control portion34, under a condition in which the oxygen partial pressure iscontrolled. Nitrogen oxide existing within the gas to be measured aroundthe periphery of the measurement electrode 62 is reduced to therebygenerate oxygen. Then, the generated oxygen is subjected to pumping bythe measurement pump cell 61. At this time, a third voltage Vp3 of thethird variable power source 68 is controlled in a manner so that thethird electromotive force V3 detected by the third oxygen partialpressure detecting sensor cell 66 becomes constant. The amount of oxygengenerated around the periphery of the measurement electrode 62 isproportional to the concentration of nitrogen oxide within the gas to bemeasured. Accordingly, the nitrogen oxide concentration within the gasto be measured can be calculated using the measurement pump current Ip3of the measurement pump cell 61. More specifically, the measurement pumpcell 61 constitutes a specified component measurement unit for measuringthe concentration of a specified component (NO) in the measurementchamber 20.

Further, the first gas sensor 10A includes an electrochemical sensorcell 70. The sensor cell 70 includes the second solid electrolyte layer28, the spacer layer 26, the first solid electrolyte layer 24, the thirdsubstrate layer 22 c, the exterior side pump electrode 44, and thereference electrode 48. In accordance with the electromotive force Vrefobtained by the sensor cell 70, it is possible to detect the oxygenpartial pressure within the gas to be measured existing externally ofthe sensor.

Furthermore, in the first sensor element 12A, a heater 72 is formed in amanner of being sandwiched from above and below between the secondsubstrate layer 22 b and the third substrate layer 22 c. The heater 72generates heat by being supplied with power from the exterior through anon-illustrated heater electrode provided on a lower surface of thefirst substrate layer 22 a. As a result of the heat generated by theheater 72, the oxygen ion conductivity of the solid electrolyte thatconstitutes the first sensor element 12A is enhanced. The heater 72 isembedded over the entire region of the preliminary adjustment chamber 21and the oxygen concentration adjustment chamber 18, and a predeterminedlocation of the first sensor element 12A can be heated and maintained ata predetermined temperature. Moreover, a heater insulating layer 74 madeof alumina or the like is formed on upper and lower surfaces of theheater 72, for the purpose of obtaining electrical insulation thereoffrom the second substrate layer 22 b and the third substrate layer 22 c(hereinafter, the heater 72, the heater electrode, and the heaterinsulating layer 74 may also be referred to collectively as a heaterportion).

In addition, the preliminary adjustment chamber 21 is driven by alater-described drive control unit 108 (see FIG. 2), and during drivingthereof, functions as a space for adjusting the oxygen partial pressurewithin the gas to be measured which is introduced from the gasintroduction port 16. The oxygen partial pressure is adjusted byoperation of a preliminary pump cell 80.

The preliminary pump cell 80 is a preliminary electrochemical pump cell,and is constituted by a preliminary pump electrode 82, which is providedsubstantially over the entirety of the lower surface of the second solidelectrolyte layer 28 facing toward the preliminary adjustment chamber21, the exterior side pump electrode 44, and the second solidelectrolyte layer 28.

Moreover, in the same manner as the main interior side pump electrode42, the preliminary pump electrode 82 is also formed using a materialthat weakens the reduction capability with respect to the NOx componentwithin the gas to be measured.

The preliminary pump cell 80, by applying a desired preliminary voltageVp0 between the preliminary pump electrode 82 and the exterior side pumpelectrode 44, is capable of pumping out oxygen within the atmosphereinside the preliminary adjustment chamber 21 into the external space, oralternatively, is capable of pumping in oxygen from the external spaceinto the preliminary adjustment chamber 21.

Further, the first gas sensor 10A includes a preliminary oxygen partialpressure detecting sensor cell 84 for controlling the preliminary pump,in order to control the oxygen partial pressure within the atmosphereinside the preliminary adjustment chamber 21. The sensor cell 84includes the preliminary pump electrode 82, the reference electrode 48,the second solid electrolyte layer 28, the spacer layer 26, and thefirst solid electrolyte layer 24.

Moreover, the preliminary pump cell 80 carries out pumping by apreliminary variable power source 86, the voltage of which is controlledbased on a preliminary electromotive force V0 detected by thepreliminary oxygen partial pressure detecting sensor cell 84.Consequently, the oxygen partial pressure within the atmosphere insidethe preliminary adjustment chamber 21 is controlled so as to become alow partial pressure that does not substantially influence themeasurement of NOx.

Further, together therewith, a preliminary pump current Ip0 thereof isused so as to control the electromotive force of the preliminary oxygenpartial pressure detecting sensor cell 84. More specifically, thepreliminary pump current Ip0 is input as a control signal to thepreliminary oxygen partial pressure detecting sensor cell 84, and bycontrolling the preliminary electromotive force V0, the gradient of theoxygen partial pressure within the gas to be measured, which isintroduced from the first diffusion rate control portion 30 into thepreliminary adjustment chamber 21, is controlled so as to remainconstant at all times.

The preliminary adjustment chamber 21 also functions as a buffer space.More specifically, it is possible to cancel fluctuations in theconcentration of the gas to be measured, which are caused by pressurefluctuations of the gas to be measured in the external space (pulsationsin the exhaust pressure, in the case that the gas to be measured is anexhaust gas of an automobile).

Furthermore, as shown schematically in FIG. 2, the first gas sensor 10Aincludes an oxygen concentration control unit 100 that controls theoxygen concentration inside the oxygen concentration adjustment chamber18, a temperature control unit 102 that controls the temperature of thefirst sensor element 12A, a specified component measurement unit 104that measures the concentration of a specified component (NO) inside themeasurement chamber 20, a preliminary oxygen concentration control unit106, a drive control unit 108, and a target component acquisition unit110.

Moreover, the oxygen concentration control unit 100, the temperaturecontrol unit 102, the specified component measurement unit 104, thepreliminary oxygen concentration control unit 106, the drive controlunit 108, and the target component acquisition unit 110 are constitutedby one or more electronic circuits having, for example, one or aplurality of CPUs (central processing units), memory devices, and thelike. The electronic circuits are software-based functional units inwhich predetermined functions are realized, for example, by the CPUsexecuting programs stored in a storage device. Of course, the electroniccircuits may be constituted by an integrated circuit such as an FPGA(Field-Programmable Gate Array), in which the plurality of electroniccircuits are connected according to the functions thereof.

In the conventional technique, after having carried out conversion intoNO with respect to all of the target components of NO and NH₃ existinginside the oxygen concentration adjustment chamber 18, the targetcomponents are introduced into the measurement chamber 20, and a totalamount of the two components is measured. Stated otherwise, it has beenimpossible to measure the concentrations of each of the two targetcomponents, that is, the respective concentrations of NO and NH₃.

In contrast thereto, as described above, by being equipped with thepreliminary adjustment chamber 21, the preliminary oxygen concentrationcontrol unit 106, the drive control unit 108, and the target componentacquisition unit 110, in addition to the oxygen concentration adjustmentchamber 18, the oxygen concentration control unit 100, the temperaturecontrol unit 102, and the specified component measurement unit 104, thefirst gas sensor 10A is made capable of measuring the respectiveconcentrations of NO and NH₃.

On the basis of the preset oxygen concentration condition, and the firstelectromotive force V1 generated in the first oxygen partial pressuredetecting sensor cell 50 (see FIG. 1), the oxygen concentration controlunit 100 feedback-controls the first variable power source 46, therebyadjusting the oxygen concentration inside the oxygen concentrationadjustment chamber 18 to a concentration in accordance with theabove-described condition.

The temperature control unit 102 feedback-controls the heater 72 on thebasis of a preset sensor temperature condition, and the measured valuefrom a temperature sensor (not shown) that measures the temperature ofthe first sensor element 12A, whereby the temperature of the firstsensor element 12A is adjusted to a temperature in accordance with theabove-described condition.

By the oxygen concentration control unit 100 or the temperature controlunit 102, or alternatively, by the oxygen concentration control unit 100and the temperature control unit 102, the first gas sensor 10A performsa control so as to convert all of the NH₃ into NO, without causingdecomposition of NO inside the oxygen concentration adjustment chamber18.

On the basis of the preset oxygen concentration condition, and thepreliminary electromotive force V0 generated in the preliminary oxygenpartial pressure detecting sensor cell 84 (see FIG. 1), the preliminaryoxygen concentration control unit 106 feedback-controls the preliminaryvariable power source 86, thereby adjusting the oxygen concentrationinside the preliminary adjustment chamber 21 to a concentration inaccordance with the condition.

By the preliminary oxygen concentration control unit 106, all of the NH₃is converted into NO, without causing decomposition of NO inside thepreliminary adjustment chamber 21.

The drive control unit 108 controls both driving and stopping of thepreliminary oxygen concentration control unit 106. Consequently, thepreliminary pump cell 80 is controlled so as to be turned on or off.During driving of the preliminary oxygen concentration control unit 106,the preliminary pump cell 80 is turned on, and therefore, all of the NH₃inside the preliminary adjustment chamber 21 is converted into NO, andflows into the oxygen concentration adjustment chamber 18 through thesecond diffusion rate control portion 32. While the preliminary oxygenconcentration control unit 106 is stopped, the preliminary pump cell 80is turned off, and therefore, the NH₃ inside the preliminary adjustmentchamber 21 is not converted into NO, but flows into the oxygenconcentration adjustment chamber 18 through the second diffusion ratecontrol portion 32.

The target component acquisition unit 110 acquires the respectiveconcentrations of NO and NH₃ on the basis of the sensor output from thespecified component measurement unit 104 at the time of driving thepreliminary oxygen concentration control unit 106, and the difference inthe sensor outputs from the specified component measurement unit 104 atthe time of stopping the preliminary oxygen concentration control unit106.

Next, processing operations of the first gas sensor 10A will bedescribed with reference also to FIGS. 3 and 4.

First, as shown in FIG. 3, the NH₃ that was introduced through the gasintroduction port 16 reaches the oxygen concentration adjustment chamber18 during a period in which the preliminary oxygen concentration controlunit 106 is stopped by the drive control unit 108. In the oxygenconcentration adjustment chamber 18, by operation of the oxygenconcentration control unit 100, a control is performed so as to convertall of the NH₃ into NO, and therefore, the NH₃ that has flowed into theoxygen concentration adjustment chamber 18 from the preliminaryadjustment chamber 21 causes an oxidation reaction of NH₃→NO to occurinside the oxygen concentration adjustment chamber 18, and all of theNH₃ inside the oxygen concentration adjustment chamber 18 is convertedinto NO. Accordingly, the NH₃ that was introduced through the gasintroduction port 16 passes through the first diffusion rate controlportion 30 and the second diffusion rate control portion 32 at a speedof the NH₃ diffusion coefficient of 2.2 cm²/sec, and after beingconverted into NO inside the oxygen concentration adjustment chamber 18,passes through the third diffusion rate control portion 34 at a speed ofthe NO diffusion coefficient of 1.8 cm²/sec, and moves into the adjacentmeasurement chamber 20.

On the other hand, during a period in which the preliminary oxygenconcentration control unit 106 is being driven by the drive control unit108, as shown in FIG. 4, the oxidation reaction of NH₃→NO occurs insidethe preliminary adjustment chamber 21, and all of the NH₃ that wasintroduced through the gas introduction port 16 is converted into NO.Accordingly, although the NH₃ passes through the first diffusion ratecontrol portion 30 at an NH₃ diffusion coefficient of 2.2 cm²/sec, afterhaving passed through the second diffusion rate control portion 32 onthe innermost side from the preliminary adjustment chamber 21, movementinto the measurement chamber 20 occurs at a speed of the NO diffusioncoefficient of 1.8 cm²/sec.

Stated otherwise, when the preliminary oxygen concentration control unit106 is switched from a stopped state into a driven state, the locationwhere the oxidation reaction of NH₃ takes place is moved from the oxygenconcentration adjustment chamber 18 to the preliminary adjustmentchamber 21.

The action of moving the location where the oxidation reaction of NH₃takes place from the oxygen concentration adjustment chamber 18 to thepreliminary adjustment chamber 21 implies that the state when the NH₃ inthe gas to be measured passes through the second diffusion rate controlportion 32 is equivalent to a state of being changed from NH₃ to NO. Inaddition, since NO and NH₃ possess different diffusion coefficients, thedifference between passing through the second diffusion rate controlportion 32 with NO or passing therethrough with NH₃ corresponds to adifference in the amount of NO that flows into the measurement chamber20, and therefore, the measurement pump current Ip3 that flows to themeasurement pump cell 61 is made to change.

In this case, the measurement pump current Ip3on when the preliminarypump cell 80 is turned on, and the amount of change ΔIp3 in themeasurement pump current Ip3off when the preliminary pump cell 80 isturned off are uniquely determined by the concentration of NH₃ in thegas to be measured. Therefore, it is possible to calculate theconcentrations of NO and NH₃ from the measurement pump current Ip3on orIp3off when the preliminary pump cell 80 is turned on or off, and theamount of change ΔIp3 in the aforementioned measurement pump currentIp3.

Accordingly, with the target component acquisition unit 110, therespective concentrations of NO and NH₃ are acquired on the basis of themeasurement pump current Ip3on when the preliminary pump cell 80 isturned on, the amount of change ΔIp3 between the measurement pumpcurrent Ip3on and the measurement pump current Ip3off when thepreliminary pump cell 80 is turned off, and the first map 112A (see FIG.2).

When shown in the form of a graph, the first map 112A becomes a graph inwhich, as shown in FIG. 5, the NH₃ concentration (ppm) within the gas tobe measured is set on the horizontal axis, and there is set on thevertical axis the difference, or in other words, the amount of changeΔIp3 between the measurement pump current Ip3on at a time that thepreliminary pump cell 80 is turned on, and the measurement pump currentIp3off at a time that the preliminary pump cell 80 is turned off. InFIG. 5, there is shown representatively a graph in which the NOconcentration converted values of the measurement pump current values,at the time that the preliminary pump cell 80 is turned off, are plottedas points pertaining to, for example, a 100 ppm system, a 50 ppm system,a 25 ppm system, and a 0 ppm system. When shown in the form of a tableto facilitate understanding, the contents thereof are as shown in FIG.6. These concentrations are obtained by experiment or by simulation.

As can be understood from FIG. 6, by using the first map 112A, and onthe basis of the measurement pump current Ip3off when the preliminarypump cell 80 is turned off (i.e., a measurement pump current valuesimilar to that of a conventional serial two-chamber type NOx sensor),any one of the 100 ppm system, the 50 ppm system, the 25 ppm system, andthe 0 ppm system is determined and used to identify the respectiveconcentrations of NO and NH₃ based on the amount of change ΔIp3.

More specifically, by specifying a point on the first map 112A from themeasurement pump current Ip3off when the preliminary pump cell 80 isturned off, and the amount of change ΔIp3, it is possible to identifythe NO concentration and the NH₃ concentration. For example, in the casethat the measurement pump current Ip3off, which is similar to that of aconventional serial two-chamber type NOx sensor, is 2.137 μA, using theaforementioned serial two-chamber type NOx sensor, it could only beunderstood that the total concentrations of NO and NH₃ is approximately100 ppm. However, in the first gas sensor 10A, by being combined withthe amount of change ΔIp3, it is possible to individually specify the NOconcentration and the NH₃ concentration, in a manner so that the NOconcentration is 100 ppm and the NH₃ concentration is 0 ppm at point p1,the NO concentration is 80 ppm and the NH₃ concentration is 17.6 ppm atpoint p2, and the NO concentration is 60 ppm and the NH₃ concentrationis 35.2 ppm at point p3. If there is no corresponding point on the firstmap 112A, the point nearest thereto may be specified, and the NOconcentration and the NH₃ concentration may be obtained, for example, bya known type of approximation calculation.

Further, the NO concentration and the NH₃ concentration may be obtainedby the following method. More specifically, as shown in theaforementioned FIG. 5, the relationship between the amount of changeΔIp3 and the NH₃ concentration is obtained beforehand by experiment orsimulation, and the NH₃ concentration is obtained from the amount ofchange ΔIp3 at a time of turning on and at a time of turning off thepreliminary pump cell 80. Then, the NO concentration may be obtained bysubtracting the NH₃ concentration, which was obtained in the foregoingmanner, from the NO concentration obtained from the sensor output at thetime that the preliminary pump cell 80 was turned off, or in otherwords, the total NO concentration obtained by converting the totalconcentrations of NO and NH₃ into NO.

Next, the process of measuring NO and NH₃ by the first gas sensor 10Awill be described with reference to the flowchart of FIG. 7.

First, in step S1 of FIG. 7, the first gas sensor 10A introduces a gasto be measured in which NO and NH₃ are mixed into the preliminaryadjustment chamber 21 through the gas introduction port 16.

In step S2, the drive control unit 108 drives the preliminary oxygenconcentration control unit 106. Consequently, the preliminary pump cell80 is turned on.

In step S3, the specified component measurement unit 104 measures the NOconcentration at the time that the preliminary pump cell 80 is turnedon. That is, the measurement pump current Ip3on is acquired. Themeasurement pump current Ip3on is input to the target componentacquisition unit 110.

In step S4, the drive control unit 108 stops driving of the preliminaryoxygen concentration control unit 106. Consequently, the preliminarypump cell 80 is turned off.

In step S5, the specified component measurement unit 104 measures the NOconcentration at the time that the preliminary pump cell 80 is turnedoff. That is, the measurement pump current Ip3off is acquired. Themeasurement pump current Ip3off is input to the target componentacquisition unit 110.

In step S6, the target component acquisition unit 110 acquires the NOconcentration and the NH₃ concentration on the basis of the measurementpump current Ip3off when the preliminary pump cell 80 is turned off, theamount of change ΔIp3 between the measurement pump current Ip3off andthe measurement pump current Ip3on when the preliminary pump cell 80 isturned on, and the first map 112A.

More specifically, the target component acquisition unit 110 specifies apoint on the first map 112A from the measurement pump current Ip3off andthe amount of change ΔIp3. In addition, the NO concentration and the NH₃concentration corresponding to the specified point are read out from thefirst map 112A, and at this time, the concentrations are set as themeasured NO concentration and the measured NH₃ concentration. If thereis no corresponding point on the first map 112A, as was discussed above,the point nearest thereto is specified, and the NO concentration and theNH₃ concentration are obtained, for example, by a known type ofapproximation calculation.

Alternatively, based on the relationship between the amount of changeΔIp3 and the NH₃ concentration shown in FIG. 5, the NH₃ concentration isobtained from the amount of change ΔIp3 at a time of turning on and at atime of turning off the preliminary pump cell 80. Then, the NOconcentration may be obtained by subtracting the NH₃ concentration,which was obtained in the foregoing manner, from the NO concentrationobtained from the sensor output at the time that the preliminary pumpcell 80 was turned off, or in other words, the total NO concentrationobtained by converting the total concentrations of NO and NH₃ into NO.

In step S7, the first gas sensor 10A determines whether or not there isa termination request (power off, maintenance, etc.) to terminate themeasurement process of NO and NH₃. If there is not a terminationrequest, the processes from step S1 and thereafter are repeated. Inaddition, in step S7, at a stage at which a termination request is made,the process of measuring NO and NH₃ in the first gas sensor 10A isbrought to an end.

In this manner, the first gas sensor 10A utilizes the first map 112A inwhich there is recorded a relationship, which is measured experimentallyin advance, between the NO concentration and the NH₃ concentrationrespectively for each of points specified by the sensor output (Ip3off)from the specified component measurement unit 104 at a time of stoppingthe preliminary oxygen concentration control unit 106, and a difference(ΔIp3) in the sensor outputs from the specified component measurementunit 104 at times of driving and stopping the preliminary oxygenconcentration control unit 106. Alternatively, as shown in FIG. 5, arelationship, which was obtained experimentally in advance, between theamount of change ΔIp3 and the NH₃ concentration may be used. Of course,such a feature may also be used in combination with the first map 112A.

In addition, the respective concentrations of NO and NH₃ are obtained bycomparing with the first map 112A the sensor output (Ip3off) from thespecified component measurement unit 104 at the time of stopping thepreliminary oxygen concentration control unit 106 during actual use, andthe difference (ΔIp3) in the sensor outputs from the specified componentmeasurement unit 104 at the times of driving and stopping thepreliminary oxygen concentration control unit 106.

Consequently, it is possible to accurately measure the respectiveconcentrations of a plurality of target components over a prolongedperiod, even under an atmosphere of a non-combusted component such asexhaust gas, and a plurality of target components (for example, NO andNH₃) that coexist in the presence of oxygen.

In addition, merely by changing the software of the control system ofthe first gas sensor 10A, the first gas sensor 10A is capable of easilyrealizing the process of measuring the respective concentrations of NOand NH₃ which heretofore could not be realized, without separatelyadding various measurement devices or the like as hardware. As a result,it is possible to improve the accuracy of controlling an NOxpurification system and detecting failures thereof. In particular, it ispossible to distinguish between NO and NH₃ in exhaust gas downstream ofan SCR system, which contributes to precisely controlling the injectedamount of urea, as well as detecting deterioration of the SCR system.

Next, a gas sensor (hereinafter referred to as a second gas sensor 10B)according to a second embodiment will be described further withreference to FIGS. 8 to 13.

As shown in FIG. 8, the second gas sensor 10B is equipped with a secondsensor element 12B having the same configuration as that of the firstsensor element 12A of the first gas sensor 10A, however, differstherefrom in that the second target component is NO₂.

Accordingly, by the oxygen concentration control unit 100 or thetemperature control unit 102, or alternatively, by the oxygenconcentration control unit 100 and the temperature control unit 102, thesecond gas sensor 10B performs a control so as to convert all of the NO₂into NO, without causing decomposition of NO inside the oxygenconcentration adjustment chamber 18.

On the basis of the preset oxygen concentration condition, and thepreliminary electromotive force V0 generated in the preliminary oxygenpartial pressure detecting sensor cell 84 (see FIG. 1), the preliminaryoxygen concentration control unit 106 feedback-controls the preliminaryvariable power source 86, thereby adjusting the oxygen concentrationinside the preliminary adjustment chamber 21 to a concentration inaccordance with the condition.

By the preliminary oxygen concentration control unit 106, all of the NO₂is converted into NO, without causing decomposition of NO inside thepreliminary adjustment chamber 21.

The drive control unit 108 controls both driving and stopping of thepreliminary oxygen concentration control unit 106. Consequently, thepreliminary pump cell 80 is controlled so as to be turned on or off.During driving of the preliminary oxygen concentration control unit 106,the preliminary pump cell 80 is turned on, and therefore, as discussedabove, all of the NO₂ inside the preliminary adjustment chamber 21 isconverted into NO, and flows into the oxygen concentration adjustmentchamber 18 through the second diffusion rate control portion 32. Whilethe preliminary oxygen concentration control unit 106 is stopped, thepreliminary pump cell 80 is turned off, and therefore, the NO₂ insidethe preliminary adjustment chamber 21 is not converted into NO, butflows into the oxygen concentration adjustment chamber 18 through thesecond diffusion rate control portion 32.

The target component acquisition unit 110 acquires the respectiveconcentrations of NO and NO₂ on the basis of the sensor output from thespecified component measurement unit 104 at the time of driving thepreliminary oxygen concentration control unit 106, and the difference inthe sensor outputs from the specified component measurement unit 104 atthe time of stopping the preliminary oxygen concentration control unit106.

Next, processing operations of the second gas sensor 10B will bedescribed with reference also to FIGS. 9 and 10.

First, as shown in FIG. 9, the NO₂ that was introduced through the gasintroduction port 16 reaches the oxygen concentration adjustment chamber18 during a period in which the preliminary oxygen concentration controlunit 106 is stopped by the drive control unit 108. In the oxygenconcentration adjustment chamber 18, by operation of the oxygenconcentration control unit 100, a control is performed so as to convertall of the NO₂ into NO, and therefore, the NO₂ that has flowed into theoxygen concentration adjustment chamber 18 from the preliminaryadjustment chamber 21 causes a decomposition reaction of NO₂→NO to occurinside the oxygen concentration adjustment chamber 18, and all of theNO₂ inside the oxygen concentration adjustment chamber 18 is convertedinto NO.

On the other hand, during a period in which the preliminary oxygenconcentration control unit 106 is being driven by the drive control unit108, as shown in FIG. 10, the decomposition reaction of NO₂→NO occursinside the preliminary adjustment chamber 21, and all of the NO₂ thatwas introduced through the gas introduction port 16 is converted intoNO.

Stated otherwise, when the preliminary oxygen concentration control unit106 is switched from a stopped state into a driven state, the locationwhere the decomposition reaction of NO₂ takes place is moved from theoxygen concentration adjustment chamber 18 to the preliminary adjustmentchamber 21.

The action of moving the location where the decomposition reaction ofNO₂ takes place from the oxygen concentration adjustment chamber 18 tothe preliminary adjustment chamber 21 implies that the state when theNO₂ in the gas to be measured passes through the second diffusion ratecontrol portion 32 is equivalent to a state of being changed from NO₂ toNO. In addition, since NO and NO₂ possess different diffusioncoefficients, the difference between passing through the seconddiffusion rate control portion 32 with NO or passing therethrough withNO₂ corresponds to a difference in the amount of NO that flows into themeasurement chamber 20, and therefore, the measurement pump current Ip3that flows to the measurement pump cell 61 is made to change.

In this case, the measurement pump current Ip3on when the preliminarypump cell 80 is turned on, and the amount of change ΔIp3 in themeasurement pump current Ip3off when the preliminary pump cell 80 isturned off are uniquely determined by the concentration of NO₂ in thegas to be measured. Therefore, it is possible to calculate theconcentrations of NO and NO₂ from the measurement pump current Ip3on orIp3off when the preliminary pump cell 80 is turned on or off, and theamount of change ΔIp3 in the aforementioned measurement pump currentIp3.

Accordingly, in the target component acquisition unit 110, therespective concentrations of NO and NO₂ are acquired on the basis of themeasurement pump current Ip3off when the preliminary pump cell 80 isturned off, the amount of change ΔIp3 between the measurement pumpcurrent Ip3off and the measurement pump current Ip3on when thepreliminary pump cell 80 is turned on, and the second map 112B (see FIG.8).

When shown in the form of a graph, the second map 112B becomes a graphin which, as shown in FIG. 11, the NO₂ concentration (ppm) within thegas to be measured is set on the horizontal axis, and there is set onthe vertical axis the difference, or in other words, the amount ofchange ΔIp3 between the measurement pump current Ip3on at a time thatthe preliminary pump cell 80 is turned on, and the measurement pumpcurrent Ip3off at a time that the preliminary pump cell 80 is turnedoff. In FIG. 11, there is shown representatively a graph in which the NOconcentration converted values of the measurement pump current values,at the time that the preliminary pump cell 80 is turned off, are plottedas points pertaining to, for example, a 500 ppm system, a 250 ppmsystem, and a 100 ppm system. When shown in the form of a table tofacilitate understanding, the contents thereof are as shown in FIG. 12.These concentrations are obtained by experiment or simulation.

As can be understood from FIG. 12, by using the second map 112B, and onthe basis of the measurement pump current Ip3off when the preliminarypump cell 80 is turned off (i.e., a measurement pump current valuesimilar to that of a conventional serial two-chamber type NOx sensor),any one of the 500 ppm system, the 250 ppm system, and the 100 ppmsystem is determined and used to identify the respective concentrationsof NO and NO₂ based on the amount of change ΔIp3.

More specifically, by specifying a point on the second map 112B from themeasurement pump current Ip3off when the preliminary pump cell 80 isturned off, and the amount of change ΔIp3, it is possible to identifythe NO concentration and the NO₂ concentration. For example, in the casethat the measurement pump current Ip3off, which is similar to that of aconventional serial two-chamber type NOx sensor, is 10.67 μA, using theaforementioned serial two-chamber type NOx sensor, could only beunderstood that the total concentrations of NO and NO₂ is approximately500 ppm. However, in the second gas sensor 10B, by being combined withthe amount of change ΔIp3, it is possible to individually specify the NOconcentration and the NO₂ concentration, in a manner so that the NOconcentration is 500 ppm and the NO₂ concentration is 0 ppm at pointp101, the NO concentration is 400 ppm and the NO₂ concentration is 116ppm at point p102, and the NO concentration is 300 ppm and the NO₂concentration is 233 ppm at point p103. If there is no correspondingpoint on the second map 112B, the point nearest thereto may bespecified, and the NO concentration and the NO₂ concentration may beobtained, for example, by a known type of approximation calculation.

Further, the NO concentration and the NO₂ concentration may be obtainedby the following method. More specifically, as shown in theaforementioned FIG. 11, the relationship between the amount of changeΔIp3 and the NO₂ concentration is obtained beforehand by experiment orsimulation, and the NO₂ concentration is obtained from the amount ofchange ΔIp3 at a time of turning on and at a time of turning off thepreliminary pump cell 80. Then, the NO concentration may be obtained bysubtracting the NO₂ concentration, which was obtained in the foregoingmanner, from the NO concentration obtained from the sensor output at thetime that the preliminary pump cell 80 was turned off, or in otherwords, the total NO concentration obtained by converting the totalconcentrations of NO and NO₂ into NO.

Next, the process of measuring NO and NO₂ by the second gas sensor 10Bwill be described with reference to the flowchart of FIG. 13.

First, in step S101 of FIG. 13, the second gas sensor 10B introduces agas to be measured in which NO and NO₂ are mixed into the preliminaryadjustment chamber 21 through the gas introduction port 16.

In step S102, the drive control unit 108 drives the preliminary oxygenconcentration control unit 106. Consequently, the preliminary pump cell80 is turned on.

In step S103, the specified component measurement unit 104 measures theNO concentration at the time that the preliminary pump cell 80 is turnedon. That is, the measurement pump current Ip3on is acquired. Themeasurement pump current Ip3on is input to the target componentacquisition unit 110.

In step S104, the drive control unit 108 stops driving of thepreliminary oxygen concentration control unit 106. Consequently, thepreliminary pump cell 80 is turned off.

In step S105, the specified component measurement unit 104 measures theNO concentration at the time that the preliminary pump cell 80 is turnedoff. That is, the measurement pump current Ip3off is acquired. Themeasurement pump current Ip3off is input to the target componentacquisition unit 110.

In step S106, the target component acquisition unit 110 acquires the NOconcentration and the NO₂ concentration on the basis of the measurementpump current Ip3off when the preliminary pump cell 80 is turned off, theamount of change ΔIp3 between the measurement pump current Ip3off andthe measurement pump current Ip3on when the preliminary pump cell 80 isturned on, and the second map 112B.

More specifically, the target component acquisition unit 110 specifies apoint on the second map 112B from the measurement pump current Ip3offand the amount of change ΔIp3. In addition, the NO concentration and theNO₂ concentration corresponding to the specified point are read out fromthe second map 112B, and at this time, the concentrations are set as themeasured NO concentration and the measured NO₂ concentration. If thereis no corresponding point on the second map 112B, in the mannerdescribed above, the point nearest thereto is specified, and the NOconcentration and the NO₂ concentration are obtained, for example, by aknown type of approximation calculation.

Alternatively, based on the relationship between the amount of changeΔIp3 and the NO₂ concentration shown in FIG. 11, the NO₂ concentrationis obtained from the amount of change ΔIp3 at a time of turning on andat a time of turning off the preliminary pump cell 80. Then, the NOconcentration may be obtained by subtracting the NO₂ concentration,which was obtained in the foregoing manner, from the NO concentrationobtained from the sensor output at the time that the preliminary pumpcell 80 was turned off, or in other words, the total NO concentrationobtained by converting the total concentrations of NO and NO₂ into NO.

In step S107, the second gas sensor 10B determines whether or not thereis a termination request (power off, maintenance, etc.) to terminate themeasurement process of NO and NO₂. If there is not a terminationrequest, the processes from step S101 and thereafter are repeated. Inaddition, in step S107, at a stage at which a termination request ismade, the process of measuring NO and NO₂ in the second gas sensor 10Bis brought to an end.

In this manner, the second gas sensor 10B utilizes the second map 112Bin which there is recorded a relationship, which is measuredexperimentally in advance, between the NO concentration and the NO₂concentration respectively for each of points specified by the sensoroutput (Ip3off) from the specified component measurement unit 104 at atime of stopping the preliminary oxygen concentration control unit 106,and a difference (ΔIp3) in the sensor outputs from the specifiedcomponent measurement unit 104 at times of driving and stopping thepreliminary oxygen concentration control unit 106. Alternatively, asshown in FIG. 11, a relationship, which was obtained experimentally inadvance, between the amount of change ΔIp3 and the NO₂ concentration maybe used. Of course, such a feature may also be used in combination withthe second map 112B.

In addition, the respective concentrations of NO and NO₂ are obtained bycomparing with the second map 112B the sensor output (Ip3off) from thespecified component measurement unit 104 at the time of stopping thepreliminary oxygen concentration control unit 106 during actual use, andthe difference (ΔIp3) in the sensor outputs from the specified componentmeasurement unit 104 at the times of driving and stopping thepreliminary oxygen concentration control unit 106.

Consequently, it is possible to accurately measure the respectiveconcentrations of a plurality of target components over a prolongedperiod, even under an atmosphere of a non-combusted component such asexhaust gas, and a plurality of target components (for example, NO andNO₂) that coexist in the presence of oxygen.

In addition, merely by changing the software of the control system ofthe second gas sensor 10B, the second gas sensor 10B is capable ofeasily realizing the process of measuring the respective concentrationsof NO and NO₂ which heretofore could not be realized, without separatelyadding various measurement devices or the like as hardware. As a result,it is possible to improve the accuracy of controlling an NOxpurification system and detecting failures thereof. In particular, it ispossible to distinguish between NO and NO₂ in exhaust gas downstream ofa DOC catalyst (Diesel Oxidation Catalyst), which contributes todetecting deterioration of the DOC catalyst.

The essence and gist of the present invention is characterized by thefollowing items (a) to (c), and the reaction by which NH₃ or NO₂ ischanged into NO can be arbitrarily selected from within a range in whicha variation in the sensor outputs can be obtained.

(a) A reaction is intentionally generated in which NH₃ or NO₂ changesinto NO before and after a diffusion rate control portion possessing apredetermined diffusion resistance.

(b) According to item (a), the concentration of NH₃ or NO₂ is determinedfrom a variation in the sensor outputs caused by a difference betweenthe diffusion coefficients of NO and NH₃, or the diffusion coefficientsof NO and NO₂.

(c) Furthermore, the NO concentration is obtained by comparing the totalconcentrations of NO and NH₃ or the total concentrations of NO and NO₂obtained by the sensor output itself with the concentration of NH₃ orNO₂ obtained due to the variation.

Next, an exhaust gas purification system 200 according to the presentembodiment which includes the first gas sensor 10A and the second gassensor 10B will be described with reference to FIGS. 14 to 19.

As shown in FIG. 14, the exhaust gas purification system 200 is a systemfor purifying exhaust gas from a combustion device 202 such as a dieselengine or the like. The exhaust gas purification system 200 includes aDOC catalyst 204 which serves to reduce hydrocarbons and carbon oxidesfrom the combustion device 202, an SCR catalyst 206 installed on adownstream side of the DOC catalyst 204, and a urea water injector 210that injects urea water, which is stored in a urea tank 208, onto theSCR catalyst 206 from an upstream side thereof. The concept of injectionalso encompasses injection by way of spraying. The combustion device 202applies energy to a load (such as a crankshaft) based on a predeterminedcombustion control performed by an ECU 212 (electronic control unit),for example.

In addition, the first sensor element 12A of the first gas sensor 10A isdisposed downstream of the SCR catalyst 206, and the second sensorelement 12B of the second gas sensor 10B is disposed between the DOCcatalyst 204 and the SCR catalyst 206, and more specifically, isinstalled between the DOC catalyst 204 and the urea water injector 210.

A first control circuit 214A which controls driving of the first gassensor 10A is connected between the ECU 212 and the first sensor element12A, and a second control circuit 214B which controls driving of thesecond gas sensor 10B is connected between the ECU 212 and the secondsensor element 12B.

The first control circuit 214A controls the above-described oxygenconcentration control unit 100 which is specialized for the first gassensor 10A, the temperature control unit 102, the specified componentmeasurement unit 104, the preliminary oxygen concentration control unit106, the drive control unit 108, and the target component acquisitionunit 110, etc.

Similarly, the second control circuit 214B controls the above-describedoxygen concentration control unit 100 which is specialized for thesecond gas sensor 10B, the temperature control unit 102, the specifiedcomponent measurement unit 104, the preliminary oxygen concentrationcontrol unit 106, the drive control unit 108, and the target componentacquisition unit 110, etc.

Further, inside the ECU 212, there are included a degree of openingcontrol unit 216 that controls the degree of opening of the urea waterinjector 210 on the basis of the NO concentration and the NH₃concentration from the first control circuit 214A, an SCR deteriorationdetection unit 218 that detects a state of degradation of the SCRcatalyst 206 based on the NO concentration and the NH₃ concentration,and a DOC deterioration detection unit 220 that detects a state ofdegradation of the DOC catalyst 204 based on the NO concentration andthe NO₂ concentration from the second control circuit 214B.

Moreover, concerning the ECU 212 as well, it is constituted by one ormore electronic circuits having, for example, one or a plurality of CPUs(central processing units), memory devices, and the like. Theabove-described degree of opening control unit 216, the SCRdeterioration detection unit 218, and the DOC deterioration detectionunit 220 are also software-based functional units in which predeterminedfunctions are realized, for example, by the CPUs executing programsstored in a storage device. Of course, the electronic circuits may beconstituted by an integrated circuit such as an FPGA, in which theplurality of electronic circuits are connected according to thefunctions thereof.

First, for the sake of comparison, a change in the NO concentration, achange in the NH₃ emission level, and a change in the SCR efficiency,which are measured with a conventional gas sensor in the case that theinjected amount of urea water is increased will be explained withreference to FIG. 15.

In FIG. 15, on the left vertical axis, there is shown the SCR efficiency(the NOx purification efficiency (%) of the SCR catalyst 206), on theright vertical axis, there is shown the NH₃ emission level (ppm) and theNO concentration (ppm) as measured by a conventional gas sensor, and onthe horizontal axis, there is shown the injected amount of urea water.In FIG. 15, the characteristic curve La indicates the NO concentration,the characteristic curve Lb indicates the SCR efficiency, and thecharacteristic curve Lc indicates the NH₃ emission level.

As can be understood from FIG. 15, by causing an increase in theinjected amount of urea water, although the SCR efficiency increases,the amount of NH₃ that is discharged also increases. Therefore, as atarget detection range Za by the gas sensor, it is preferable to set alower limit of the SCR efficiency to 90%, and to set an upper limit ofthe NH₃ emission level to 10 ppm.

However, due to the influence of interference caused by an increase inthe NH₃ emission level, the sensitivity of the gas sensor (the variationwidth ΔNO of the measurement value of the gas sensor with respect to anincremental width of the urea water injection amount) becomes small in aregion in which the SCR efficiency is greater than or equal to 90%,while additionally, since an error component Er is also includedtherein, a problem results in that a precise control of the injectedamount of urea water cannot be assured.

On the other hand, FIG. 16 shows a change in the sensor output of thefirst gas sensor 10A, a change in the NH₃ emission level, and a changein the SCR efficiency, in the case that the injected amount of ureawater is increased. In FIG. 16, the SCR efficiency (%) is shown on theleft vertical axis, the NH₃ emission level (ppm) and the sensor outputIp3 (μA) of the first gas sensor 10A are shown on the right verticalaxis, and the injected amount of urea water is shown on the horizontalaxis. In FIG. 16, the characteristic curve Ld indicates the sensoroutput, the characteristic curve Lb indicates the SCR efficiency, andthe characteristic curve Lc indicates the NH₃ emission level.

In the first gas sensor 10A, the sensor output Ip3 from the specifiedcomponent measurement unit 104 undergoes variations corresponding todriving and stopping of the preliminary oxygen concentration controlunit 106, and more specifically, responsive to the preliminary pump cell80 being turned on and off. The variation (ΔIp3) in the sensor outputIp3 increases as the concentration of NH₃ increases. Accordingly, asdescribed above, the respective concentrations of NO and NH₃ areacquired on the basis of the measurement pump current Ip3on when thepreliminary pump cell 80 is turned on, the amount of change ΔIp3 betweenthe measurement pump current Ip3on and the measurement pump currentIp3off when the preliminary pump cell 80 is turned off, and the firstmap 112A.

Conventionally, the concentrations of NO and NH₃ have been measuredusing only the sensor output when NH₃ is subjected to an oxidationreaction and is converted into NO, without causing decomposition of NOin the oxygen concentration adjustment chamber 18. In contrast thereto,in the first gas sensor 10A, in addition to the sensor output Ip3offobtained when NH₃ is directly introduced into the oxygen concentrationadjustment chamber 18 without the NH₃ being subjected to an oxidationreaction, and without causing decomposition of NO in the preliminaryadjustment chamber 21, the NO concentration and the NH₃ concentrationare acquired from the first map 112A on the basis of the amount ofchange ΔIp3. The amount of change ΔIp3 is indicative of an amount ofchange between the sensor output Ip3off and the sensor output Ip3on whenthe NH₃ is subjected to the oxidation reaction without causingdecomposition of NO in the preliminary adjustment chamber 21.

Therefore, the concentration corresponding to the sensor output of thefirst gas sensor 10A can be divided into an NH₃ concentration (aconcentration corresponding to the amount of change ΔIp3), and an NOconcentration (a concentration corresponding to the concentration andthe amount of change ΔIp3 corresponding to the sensor output of thefirst gas sensor 10A).

Therefore, as for the target detection range Za by the first gas sensor10A, as described above, the lower limit of the SCR efficiency is set to90%, and the upper limit of the NH₃ discharge amount is set to 10 ppm,and for example, even if the variation width of the sensor output Ip3offis small, it is possible to accurately acquire the NO concentration andthe NH₃ concentration.

As a result, assuming that the injected amount of urea water is adjustedin a manner so that the NH₃ concentration and the NO concentration arerespectively less than or equal to predetermined individualconcentrations, the NOx purification system can be accuratelycontrolled.

Next, the relationship between the injected amount of urea water and thesensor output of the first gas sensor 10A will be described withreference to FIGS. 17A and 17B.

FIG. 17A shows a relationship between an excess or deficiency in theinjected amount of urea water and the sensor output of the first gassensor 10A. In the region where the injected amount of urea water isless than an equivalent point, since all of the NH₃ produced by theinjection of urea water is consumed by the decomposition of NOx, thereis almost no outflow of NH₃. Therefore, the sensor output of the firstgas sensor 10A is substantially the same as the sensor output when thepreliminary pump cell 80 is turned on and the sensor output when thepreliminary pump cell 80 is turned off, and as indicated by the solidline Ld1 in FIG. 17A, decreases linearly as the injected amount of ureawater increases. In addition, the sensor output becomes lowest at theequivalent point.

On the other hand, when the injected amount of urea water exceeds theequivalent point, since an excessive amount of urea remains in the formof NH₃, the residual NH₃ emission level is detected as the amount ofchange ΔIp3 between the sensor output when the preliminary pump cell 80is turned on and the sensor output when the preliminary pump cell 80 isturned off. More specifically, as indicated by the solid line Ld2 inFIG. 17A, the sensor output of the first gas sensor 10A exhibits arectangular shape. In addition, the amount of change ΔIp3 increasesaccompanying an increase in the outflow amount of NH₃.

The sensor output of the first gas sensor 10A, which is shown in FIG.17A, can be separated into a sensor output (NO output) in regards to NO,and a sensor output (NH₃ output) in regards to NH₃, as shown in FIG.17B. In this case, the NO output decreases linearly toward theequivalent point in the region where the injected amount of urea wateris deficient. In addition, the NO output becomes a lowest value at theequivalent point of the urea water inflow amount, and is maintained atthe lowest value in the region where the injected amount of urea wateris excessive.

In a contrary manner to the NO output, the NH₃ output exhibits a lowestvalue at the equivalent point and in the region where the injectedamount of urea water is deficient, and in a region where the injectedamount of urea water is excessive, exhibits an output corresponding tothe concentration of NH₃ generated by the excess urea.

By controlling the degree of opening of the urea water injector 210(hereinafter referred to as the degree of opening of the urea injector)utilizing the change in the NO output and the NH₃ output with respect tothe injected amount of urea water, it is possible to control the exhaustgas purification system 200 under a condition in which the NOxpurification efficiency is higher.

As an example, a description will be given with reference to FIGS. 18Ato 18C concerning the control of the degree of opening of the ureainjector using changes in NO output (NO concentration) and NH₃ output(NH₃ concentration). FIG. 18A is a graph showing changes in the NOoutput accompanying an elapse of time, FIG. 18B is a graph showingchanges in the NH₃ output accompanying an elapse of time, and FIG. 18Cis a graph showing changes in the degree of opening of the urea injectoraccompanying an elapse of time.

First, the degree of opening of the urea injector undergoes expansionfrom time t0 when the NO output (NO concentration) has reached a firstthreshold value Th1. The first threshold value Th1 is set to a valuethat is higher than the NO output at the respective equivalent points ofthe NO output and the NH₃ output.

The NO output decreases until time t1 when the injected amount of ureawater reaches the equivalent point, and the NO output is maintained atthe minimum value even after having passed through the equivalent point.

The NH₃ output (NH₃ concentration) starts to increase from time t1 uponhaving passed through the equivalent point, and therefore, the degree ofopening of the urea injector starts to be restricted at time t2 when theNH₃ output reaches a second threshold value Th2. The second thresholdvalue Th2 is set to a value that is higher than the NH₃ output at therespective equivalent points of the NO output and the NH₃ output. Forexample, it is set to a value at which the NH₃ output corresponds to 2to 10 ppm.

When the degree of opening of the urea injector is continuouslythrottled, the NH₃ output begins to decrease, the NH₃ output reaches aminimum value at time t3 upon having reached the equivalent point, andis maintained at the minimum value even after having passed through theequivalent point.

The NO output starts to increase from time t3 upon having passed throughthe equivalent point, and thereafter reaches the first threshold valueTh1 at time t4. Hence, the degree of opening of the urea injector startsto be expanded at time t4. Thereafter, since the control operations arethe same as those after time t0, explanation of such operations will beomitted.

As described above, in the first gas sensor 10A, it is possible todistinguish between NO and NH₃ in exhaust gas downstream of the SCRcatalyst 206. In this case, it is effective to measure an NO/NH₃ ratioin order to detect deterioration of the SCR catalyst 206. Accordingly,as shown in FIG. 14, in the SCR deterioration detection unit 218 insidethe ECU 212, the NO/NH₃ ratio is calculated based on the NOconcentration and the NH₃ concentration from the first control circuit214A, whereby it is possible to detect the deterioration of the SCRcatalyst 206. Information concerning the deterioration of the SCRcatalyst 206 is displayed, for example, through a display device 222.Further, even if NO₂ is present in the exhaust gas downstream of the SCRcatalyst 206, by using a correction value obtained experimentally or anempirical correction value, it is possible to control the SCR catalystsystem so as to be substantially free of problems.

Similarly, in the second gas sensor 10B, it is possible to distinguishbetween NO and NO₂ in exhaust gas downstream of the DOC catalyst 204. Inthis case, upon initial degradation (a reduction in oxidizing capacity)of the DOC catalyst 204, a change in the NO/NO₂ ratio (reduction in NO₂)is more conspicuous than an increase in the emitted amount ofnon-combusted components such as HC or the like. Accordingly, in the DOCdeterioration detection unit 220 inside the ECU 212, the NO/NO₂ ratio iscalculated based on the NO concentration and the NO₂ concentration fromthe second control circuit 214B, whereby it is possible to detect thedeterioration of the DOC catalyst 204. Information concerning thedeterioration of the DOC catalyst 204 is displayed, for example, throughthe display device 222.

Next, processing operations of the exhaust gas purification system 200according to the present embodiment will be described with reference tothe flowchart of FIG. 19.

First, in step S201, the degree of opening control unit 216 determineswhether or not the NO output (NO concentration) from the first controlcircuit 214A has reached the first threshold value Th1. If the firstthreshold value Th1 has been reached, the process proceeds to step S202,and the degree of opening control unit 216 controls the degree ofopening of the urea injector in a direction to open as time passes.

If it is determined in step S201 that the NO output (NO concentration)has not reached the first threshold value Th1, the process proceeds tostep S203, and the degree of opening control unit 216 determines whetheror not the NH₃ output (NH₃ concentration) from the first control circuit214A has reached the second threshold value Th2. If the second thresholdvalue Th2 has been reached, the process proceeds to step S204, and thedegree of opening control unit 216 controls the degree of opening of theurea injector in a direction to close as time passes.

At a stage at which processing by the aforementioned step S202 or stepS204 is ended, or in the case it is determined in step S203 that the NH₃output (NH₃ concentration) has not reached the second threshold valueTh2, the process proceeds to the following step S205, whereupon thedegree of opening control unit 216 determines whether or not the degreeof opening of the urea injector has reached a predetermined degree ofopening, for example, a degree of opening that is ¾ of being totallyopened (referred to as a ¾ opening degree). If the degree of opening hasreached the ¾ opening degree, the process proceeds to step S206,whereupon the SCR deterioration detection unit 218 calculates the NO/NH₃ratio on the basis of the NO concentration and the NH₃ concentrationfrom the first control circuit 214A.

At a stage at which processing by the aforementioned step S206 is ended,or in the case it is determined in step S205 that the degree of openingof the urea injector has not reached the ¾ opening degree, then in thefollowing step S207, the SCR deterioration detection unit 218 displaysinformation concerning the deterioration of the SCR catalyst 206 inaccordance with the calculation result on the display device 222. Forexample, if the calculation result exceeds 1, a message is displayedindicating that the SCR catalyst 206 is deteriorated, and if thecalculation result is less than or equal to 1, a message is displayedindicating that the SCR catalyst 206 is not deteriorated.

Thereafter, in step S208, in the DOC deterioration detection unit 220,the NO/NO₂ ratio is calculated based on the NO concentration and the NO₂concentration from the second control circuit 214B. Thereafter, in stepS209, the DOC deterioration detection unit 220 displays informationconcerning the deterioration of the DOC catalyst 204 in accordance withthe calculation result on the display device 222. For example, if thecalculation result exceeds 1, a message is displayed indicating that theDOC catalyst 204 is deteriorated, and if the calculation result is lessthan or equal to 1 (in most cases, it equals 1), a message is displayedindicating that the DOC catalyst 204 is not deteriorated.

Thereafter, in step S210, a determination is made as to whether or notthere is a termination request (power off, maintenance, etc.) withrespect to the exhaust gas purification system 200. If there is not atermination request, step S201 is returned to, and the processes fromstep S201 and thereafter are repeated. If there is a terminationrequest, the processing carried out by the exhaust gas purificationsystem 200 is brought to an end.

Normally, control of an SCR system and a method for carrying out failurediagnosis can be contemplated by attaching on a downstream side of theSCR catalyst a conventional two-chamber type NOx sensor and an NH₃sensor in which a change in resistance of an oxide semiconductorelectrode or a mixed potential is used, and by separately measuring therespective components.

However, due to differences in the sensitivity and response speeds ofthe respective sensors, as well as differences in the deterioration overtime of the respective sensors, it has not been possible to accuratelycontrol the injected amount of urea water, or to detect deterioration ofthe SCR catalyst 206 over a prolonged time period.

Such a disadvantage also applies to detecting deterioration of the DOCcatalyst 204, in which a method for carrying out failure detection ofthe DOC catalyst 204 can be contemplated by attaching on a downstreamside of the DOC catalyst 204 a conventional two-chamber type NOx sensorand an NO₂ sensor in which a change in resistance of an oxidesemiconductor electrode or a mixed potential is used, and by separatelymeasuring the respective components.

In the exhaust gas purification system 200 according to the presentembodiment, the first gas sensor 10A is used, which is capable ofreliably detecting a difference in diffusion coefficients, even for anextremely unstable component such as NH₃, and can detect the respectiveconcentrations of NO and NH₃ with a single sensor element.

More specifically, by controlling the injected amount of urea water anddetecting deterioration of the SCR catalyst 206 on the basis of outputs,i.e., the NO concentration and the NH₃ concentration, from the singlefirst gas sensor 10A, no adverse influence is received due to variationsin output between individual sensors made up from a combination of aserial two-chamber type NOx sensor and another sensor. Furthermore,since no adverse influence is received due to variations betweenrespective individual sensor outputs over time, purification of NOx andsuppression of the NH₃ emission level can be carried out with highaccuracy over a prolonged time period.

In addition, since deterioration of the DOC catalyst 204 is detected onthe basis of outputs, i.e., the NO concentration and the NO₂concentration, from the single second gas sensor 10B, it is possible todetect deterioration of the DOC catalyst 204 accurately over a prolongedtime period.

The exhaust gas purification system and the exhaust gas purificationmethod according to the present invention are not limited to theembodiments described above, and it is a matter of course that variousconfigurations could be adopted therein without deviating from theessence and gist of the present invention.

In the example discussed above, the measurement chamber 20 is disposedadjacent to the auxiliary adjustment chamber 18 b, and the measurementelectrode 62 is arranged inside the measurement chamber 20. However, asshown in FIG. 20, the measurement electrode 62 may be arranged insidethe auxiliary adjustment chamber 18 b, and may be formed of a ceramicporous body such as alumina (Al₂O₃) serving as the third diffusion ratecontrol portion 34 so as to cover the measurement electrode 62. In thiscase, the surrounding periphery of the measurement electrode 62functions as the measurement chamber 20.

Further, in the above example, NH₃ or NO₂ as the second target componentis converted into NO inside the preliminary adjustment chamber 21 at aconversion ratio of 100%. However, the conversion ratio of NH₃ or NO₂need not necessarily be 100%, and the conversion ratio can be setarbitrarily, within a range in which a correlation with goodreproducibility with the NH₃ concentration or the NO₂ concentrationwithin the gas to be measured is obtained.

Further, driving of the preliminary oxygen concentration control unit106 may be performed in a direction of pumping oxygen out from theinterior of the preliminary adjustment chamber 21, or in a direction ofpumping oxygen into the preliminary adjustment chamber 21, and it issufficient insofar as the measurement pump current Ip3, which is theoutput of the measurement pump cell 61, changes with goodreproducibility due to the presence of NH₃ or NO₂ that serves as thesecond target component.

Furthermore, driving of the preliminary oxygen concentration controlunit 106 may turn on or off the application of a constant voltage, ormay turn on or off the application of a variable voltage based on theoxygen concentration inside the preliminary adjustment chamber 21.

In addition, the driven time period and the stopped time period of thepreliminary oxygen concentration control unit 106 can be arbitrarily setdepending on a desired detection accuracy of the first target componentand the second target component.

In practicing the present invention, various configurations forimproving reliability may be added as components for an automobile tosuch an extent that the concept of the present invention is notimpaired.

The invention claimed is:
 1. An exhaust gas purification systemincluding: a urea water injector disposed between a combustion deviceand a selective reduction type catalyst, the urea water injector havingopen and closed positions; a first gas sensor disposed downstream of theselective reduction type catalyst and adapted to detect an NOconcentration and an NH3 concentration in the exhaust gas downstream ofthe selective reduction type catalyst; and a controller configured tocontrol a degree of opening of the urea water injector between the openand closed positions, wherein the controller is configured to controlthe degree of opening toward the open position as time passes upon theNO concentration reaching a predetermined first threshold value andcontrol the degree of opening toward the closed position as time passesupon the NH3 concentration reaching a predetermined second thresholdvalue, wherein the first gas sensor element includes: a sensor elementhaving a structural body made up from a solid electrolyte that exhibitsoxygen ion conductivity, a gas introduction port formed in thestructural body and into which a gas to be measured is introduced, anoxygen concentration adjustment chamber formed in the structural bodyand communicating with the gas introduction port, and a measurementchamber formed in the structural body and communicating with the oxygenconcentration adjustment chamber; and a processor coupled to a memorystoring instructions that when executed by the processor configure theprocessor to: control the oxygen concentration in the oxygenconcentration adjustment chamber; control a temperature of the sensorelement; and measure a concentration of a specified component in themeasurement chamber, the first gas sensor further comprising: apreliminary adjustment chamber provided within the structural bodybetween the gas introduction port and the oxygen concentrationadjustment chamber and a preliminary cell that pumps oxygen in and outof the preliminary adjustment chamber, the preliminary adjustmentchamber communicating with the gas introduction port, wherein theprocessor is further configured to: control the oxygen concentrationinside the preliminary adjustment chamber by driving the preliminarypump cell to pump oxygen in and out of the preliminary adjustmentchamber; control the driving and stopping of the preliminary pump cell;and acquire the NO concentration and the NH3 concentration, on a basisof a difference between a sensor output upon the preliminary pump cellbeing driven, and a sensor output upon the preliminary pump cell beingstopped, and one of the respective sensor outputs.
 2. The exhaust gaspurification system according to claim 1, wherein the first thresholdvalue is set to a value that is higher than the NO concentration atrespective equivalent points of the NO concentration and the NH₃concentration, and the second threshold value is set to a value that ishigher than the NH₃ concentration at the respective equivalent points.3. The exhaust gas purification system according to claim 1, furtherincluding the controller being configured to detect a condition ofdeterioration of the selective reduction type catalyst, by comparing aratio (NO/NH₃) of the NO concentration to the NH₃ concentration, at apredetermined degree of opening, with a prescribed value and judgedeterioration of the selective reduction type catalyst upon the ratioexceeding the prescribed value.
 4. The exhaust gas purification systemaccording to claim 1, wherein a first map, in which there is recorded arelationship, which is measured experimentally in advance, between theNO concentration and the NH3 concentration respectively for each ofpoints specified by the sensor output at a time of stopping thepreliminary pump cell, and a difference in the sensor outputs at timesof driving and stopping the preliminary pump cell, is stored in thememory, and wherein the processor is further configured to obtain therespective concentrations of NO and NH3 by comparing with the first mapthe sensor outputs at the time of stopping the preliminary pump cellduring actual use, and the difference in the sensor outputs at the timesof driving and stopping the preliminary pump cell.
 5. The exhaust gaspurification system according to claim 1, wherein the processor isconfigured to obtain the NH3 concentration corresponding to a differencein the sensor outputs at times of driving and stopping the preliminarypump cell during actual use, on the basis of a relationship, which ismeasured experimentally in advance, between the NH3 concentration andthe difference in the sensor outputs at the times of driving andstopping the preliminary pump cell, and wherein and the processor isfurther configured to obtain the NO concentration by subtracting the NH3concentration, which was obtained beforehand from the difference in thesensor outputs, from a total NO concentration in which all of theconcentrations of NO and NH3 obtained from the sensor output at a timeof stopping the preliminary pump cell are converted into NO.
 6. Theexhaust gas purification system according to claim 1, further including:an oxidation catalyst disposed between the combustion device and theurea water injector; a second gas sensor disposed between the oxidationcatalyst and the urea water injector, wherein the second gas sensor isconfigured to detect an NO concentration and an NO₂ concentration in anexhaust gas downstream of the oxidation catalyst; and wherein theprocessor is further configured to detect a deterioration condition ofthe oxidation catalyst by comparing a ratio of the NO concentration tothe NO₂ concentration (NO/NO₂) with a second prescribed value, and judgedeterioration of the oxidation catalyst upon the ratio exceeding thesecond prescribed value.
 7. The exhaust gas purification systemaccording to claim 6, wherein the second gas sensor includes: a sensorelement having a structural body made up from a solid electrolyte thatexhibits oxygen ion conductivity, a gas introduction port formed in thestructural body and into which a gas to be measured is introduced, anoxygen concentration adjustment chamber formed in the structural bodyand communicating with the gas introduction port, and a measurementchamber formed in the structural body and communicating with the oxygenconcentration adjustment chamber; and a processor coupled to a memorystoring instructions that when executed by the processor configure theprocessor to: control the oxygen concentration in the oxygenconcentration adjustment chamber; control a temperature of the sensorelement; and measure a concentration of a specified component in themeasurement chamber, the second gas sensor further comprising: apreliminary adjustment chamber provided within the structural bodybetween the gas introduction port and the oxygen concentrationadjustment chamber and a preliminary pump cell that pumps oxygen in andout of the preliminary adjustment chamber, the preliminary adjustmentchamber communicating with the gas introduction port, wherein theprocessor is further configured to: control the oxygen concentrationinside the preliminary adjustment chamber by driving the preliminarypump cell to pump oxygen in and out of the preliminary adjustmentchamber; control driving and stopping of the preliminary pump cell; andacquire the NO concentration and the NH₃ concentration, on a basis of adifference between a sensor output upon the preliminary pump cell beingdriven, and a sensor output upon the preliminary pump cell beingstopped, and one of the respective sensor outputs.
 8. The exhaust gaspurification system according to claim 7, wherein a second map, in whichthere is recorded a relationship, which is measured experimentally inadvance, between the NO concentration and the NO₂ concentrationrespectively for each of points specified by the sensor output at a timeof stopping the preliminary pump cell, and a difference in the sensoroutputs at times of driving and stopping the preliminary pump cell isstored in the memory, and wherein the processor is further configured toobtain the respective concentrations of NO and NO₂ by comparing with thesecond map the sensor output at the time of stopping the preliminarypump cell during actual use, and the difference in the sensor outputs atthe times of driving and stopping the preliminary pump cell.
 9. Theexhaust gas purification system according to claim 7, wherein theprocessor is configured to obtain the NO₂ concentration corresponding toa difference in the sensor outputs at times of driving and stopping thepreliminary pump cell during actual use, on a basis of a relationship,which is measured experimentally in advance, between the NO₂concentration and the difference in the sensor outputs at the times ofdriving and stopping the preliminary pump cell, and wherein theprocessor is further configured to obtain the NO concentration bysubtracting the NO₂ concentration, which was obtained beforehand fromthe difference in the sensor outputs, from a total NO concentration inwhich all of the concentrations of NO and NO₂ obtained from the sensoroutput at a time of stopping the preliminary pump cell are convertedinto NO.
 10. An exhaust gas purification method for purifying an exhaustgas using an exhaust gas purification system, the exhaust gaspurification system including: a urea water injector disposed between acombustion device and a selective reduction type catalyst, the ureawater injector having open and closed positions; a first gas sensordisposed downstream of the selective reduction type catalyst and adaptedto detect an NO concentration and an NH3 concentration in an exhaust gasdownstream of the selective reduction type catalyst; and a controllerconfigured to control a degree of opening of the urea water injectorbetween the open and closed positions, including the steps of:controlling the degree of opening toward the open position as timepasses upon the NO concentration reaching a predetermined firstthreshold value; and controlling the degree of opening toward the closedposition as time passes upon the NH3 concentration reaching apredetermined second threshold value, wherein the first gas sensorelement includes: a sensor element having a structural body made up froma solid electrolyte that exhibits oxygen ion conductivity, a gasintroduction port formed in the structural body and into which a gas tobe measured is introduced, an oxygen concentration adjustment chamberformed in the structural body and communicating with the gasintroduction port, and a measurement chamber formed in the structuralbody and communicating with the oxygen concentration adjustment chamber;and a processor coupled to a memory storing instructions that whenexecuted by the processor configure the processor to: control the oxygenconcentration in the oxygen concentration adjustment chamber; control atemperature of the sensor element; and measure a concentration of aspecified component in the measurement chamber, the first gas sensorfurther comprising: a preliminary adjustment chamber provided within thestructural body between the gas introduction port and the oxygenconcentration adjustment chamber and a preliminary cell that pumpsoxygen in and out of the preliminary adjustment chamber, the preliminaryadjustment chamber communicating with the gas introduction port, whereinthe processor is further configured to: control the oxygen concentrationinside the preliminary adjustment chamber by driving the preliminarypump cell to pump oxygen in and out of the preliminary adjustmentchamber; control the driving and stopping of the preliminary pump cell;and acquire the NO concentration and the NH3 concentration, on a basisof a difference between a sensor output upon the preliminary pump cellbeing driven, and a sensor output upon the preliminary pump cell beingstopped, and one of the respective sensor outputs.
 11. The exhaust gaspurification method according to claim 10, further including the step ofcomparing a ratio of the NO concentration to the NH₃ concentration(NO/NH₃) at a predetermined degree of opening with a prescribed value,and judging deterioration of the selective reduction type catalyst uponthe ratio exceeding the prescribed value.
 12. The exhaust gaspurification method according to claim 10, wherein the exhaust gaspurification system includes: an oxidation catalyst disposed between thecombustion device and the urea water injector; and a second gas sensorconfigured to detect an NO concentration and an NO₂ concentration in anexhaust gas downstream of the oxidation catalyst, and the methodincludes the step of: comparing a ratio of the NO concentration to theNO₂ concentration (NO/NO₂) with a second prescribed value, and judgingdeterioration of the oxidation catalyst upon the ratio exceeding thesecond prescribed value.